Chromatic dispersion compensated optical fiber communication system

ABSTRACT

An optical fiber communication system according to the invention comprises, in addition to conventional single mode (SM) optical fiber, dispersion compensating (DC) optical fiber. The DC fiber is selected such that its chromatic dispersion (D DC  (λ)) and first derivative of the chromatic dispersion with respect to wavelength (D&#39; DC  (λ)) at a wavelength λ=λ op  have opposite sign from those of the SM fiber. Advantageously, the DC fiber is selected such that L DC  ·D DC  (λ)+L·D(λ) is approximately zero at λ=λ op , and such that D&#39; DC  (λ) is approximately equal to --(L/L DC )·D&#39;(λ) at λ=λ op , where L DC  and L are appropriate lengths of DC and SM fiber, respectively. Typically, L is much greater than L DC , and λ op  typically is about 1.5 μm, e.g., about 1.56 μm. Preferred embodiments of the invention have low chromatic dispersion over a substantial wavelength region (e.g., dispersion less than 1.5 ps/nm·km over at least 50 nm) that includes λ o , facilitating wavelength division multiplexing.

This is a continuation-in-part application of U.S. patent applicationSer. No. 07/766,600, filed Sep. 26, 1991 now U.S. Pat. No. 5,185,827.

TECHNICAL FIELD

This invention relates to optical fiber communication systems and, moreparticularly, to systems that comprise chromatic dispersion compensatingoptical fiber.

BACKGROUND OF THE INVENTION

With increased demand being placed on existing optical fiber facilities,lightwave communications providers are looking for ways to increase theusable bandwidth available for customers from existing fiber withoutinstalling additional fibers. Lightwave communication systems depend onoptical fiber to transport the lightwave signals from one location toanother in the system.

Optical fiber, both single mode and multimode, has modal and chromaticdispersion parameters which result from material and waveguidecharacteristics of the fiber. Chromatic dispersion causes lightwaves atone wavelength to travel at a different velocity through the opticalfiber than lightwaves at another wavelength. Thus, for example, a shortpulse input to one end of the fiber emerges from the far end as abroader pulse. Pulse broadening effects and, therefore, dispersion limitthe rate at which information can be carried through an optical fiber.

Several solutions have been proposed to avoid or at least mitigate theeffects of dispersion. These solutions include dispersion compensationtechniques. One approach described in U.S. Pat. No. 4,261,639, involvesthe interconnection of two optical fibers having appropriate lengths andhaving opposite group velocity dispersion characteristics so that thetotal dispersion in one fiber is substantially matched and canceled bythe total dispersion in the connected fiber. While this technique offersa possible solution to the dispersion problem, it is impractical (1)because the length of compensating (opposite dispersion) fiber typicallywill be very long, on the order of the length of the existing installedfiber, and (2) because there may be insufficient fiber available havingthe appropriate dispersion of opposite sign to dispersion of theexisting installed fiber.

In order to avoid the use of dispersion compensating fibers as describedabove, reflective and transmissive Fabry-Perot etalon structures havebeen proposed for providing optical equalization. For a discussion ofthese structures, see J. of Lightwave Technology, Vol. 8, No. 5, pp.649-59 (1990). Adaptive control via a feedback loop is demonstrated forthe structures. It is believed that the hardware complexity of theseoptical equalization structures together with the need to providesubstantially continuous monitoring and tuning of the etalonsignificantly affect the commercial attractiveness of such structures.Thus, a convenient dispersion compensation technique, especially such atechnique that results in low chromatic dispersion over a significantwavelength range, would be of substantial interest. This applicationdiscloses such a technique.

SUMMARY OF THE INVENTION

Chromatic dispersion is efficiently compensated in a compact device bycombining a spatial mode converter with a dispersive waveguide, such asa length of multimode or dual-mode fiber, having a dispersioncharacteristic of substantially equivalent magnitude, and opposite sign,to the desired amount of dispersion to be compensated. The spatial modeconverter exchanges the optical energy from one propagating spatial modeto another spatial mode whose propagation is supported by the multimodeor dual-mode fiber. By performing the mode exchange, it is possible toutilize the larger opposite dispersion in the dispersive waveguide tobalance the unwanted chromatic dispersion. This apparatus is applicableto lightwave repeaters, regenerators, transmitters, and receivers.

In one embodiment, the dispersive waveguide and the spatial modeconverter are juxtaposed so that light is coupled into the higher orderspatial mode supported in the dual-mode fiber, subjected to dispersioncompensation (pulse compression), and transferred to a spatial modeconverter for conversion to the fundamental spatial mode to betransmitted in a single mode transmission fiber. This embodimentpredistorts the light in a way which substantially cancels the effectsof dispersion in the long transmission fiber.

Still other embodiments include the use of spatial mode converters oneither end of the dual-mode fiber, and the use of an absorption elementin the dual-mode waveguide or fiber to substantially eliminatepropagation of any unwanted spatial mode or modes in the dual-mode fiberor waveguide.

A further embodiment is an article, typically an optical fibercommunication system, that comprises dispersion compensation (DC) fiberas well as conventional single mode fiber, with the former havingnegative chromatic dispersion and a negative derivative with respect towavelength at a wavelength λ_(op), typically about 1.5 μm. Moregenerally, the DC fiber has chromatic dispersion and dispersion slope ofopposite sign as the chromatic dispersion and dispersion slope of thesingle mode fiber.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the invention may be obtained byreading the following description of specific illustrative embodimentsof the invention in conjunction with the appended drawing in which:

FIGS. 1-3 show various block diagram embodiments of the dispersioncompensation apparatus;

FIGS. 4-5 show graphs of optical pulse power versus time for mode-lockedpulses at different wavelengths and at different locations within thedispersion compensation apparatus;

FIGS. 6-8 show alternative prior art embodiments for the optical modeconverter shown in FIGS. 1-3;

FIGS. 9-10 show illustrative cross-sectional views of rare-earth dopedoptical fiber used in realizing specific embodiments of the dispersivewaveguide element shown in FIGS. 1-3; and

FIG. 11 schematically shows an exemplary optical fiber communicationsystem according to the invention.

DETAILED DESCRIPTION

Information capacity represented as the bit rate of data signals is animportant measure of capability of a telecommunication system. Becauseof certain effects along the transmission medium of the system, the bitrate and, therefore, the utility and appeal of the system are seriouslyimpaired. This is especially true in lightwave communication systemswherein optical fibers have an essentially unlimited informationcapacity which is practically unattainable because of the dispersiveeffects in optical fibers.

In accordance with the principles of the present invention, fiberchromatic dispersion may be compensated or equalized to substantiallyremove its effect on propagating lightbeams or lightwave signals. It iscontemplated that compensation be performed on the lightwave signalbefore, after, or even during its exposure to the fiber dispersion inthe communication system. Several different embodiments of the inventionare described below and shown in the drawing to illustrate variousconditions for its use.

Exemplary elements for practicing the present invention are a modeconverter and a dispersive waveguide. The mode converter selectivelyexcites a prescribed spatial mode for a lightwave signal so that thespatial mode output by the mode converter is different from the spatialmode input to the mode converter. The dispersive waveguide induceschromatic dispersion in a higher order spatial mode than the fundamentalspatial mode wherein the total dispersion is substantially equal inmagnitude, and opposite in sign to the desired amount of dispersion tobe compensated. Total dispersion (ps/nm) is equal to the chromaticdispersion (ps/nm·km) times the length of the dispersive waveguide (km).By introducing dispersion in the higher order spatial mode of alightwave signal, it is possible to provide dispersion compensation in arelatively compact device.

Spatial modes of a lightwave signal are mentioned throughout thisdescription. For most lightwave systems which utilize single-modeoptical fiber, the usual spatial mode supported by the fiber andwaveguides in the system is the fundamental or lowest order spatialmode, LP₀₁. Higher order spatial modes such as the LP₁₁ spatial mode andothers may, in an exemplary embodiment of the invention, be supported inthe optical mode converter and dispersive waveguide element.

Guiding properties of all waveguides show an enhanced sensitivity towavelength of operation when the wavelength of operation is selected tobe close to the cut-off wavelength for a given spatial mode. This istypically manifested by a wavelength-dependent change of the propagationconstant for the spatial mode near cut-off. For example, in an opticalfiber this effect is seen for the LP₁₁ spatial mode near its cut-offwavelength and is manifested as an ever-increasing waveguide dispersionas the cut-off wavelength for the spatial mode is approached. It will beunderstood by those persons skilled in the art that the wavelength ofoperation (λ_(op)) generally is selected to be less than the cut-offwavelength (λ_(c)) and that the cut-off wavelength is the wavelengthabove which the particular spatial mode does substantially not propagatein the waveguide.

An understanding of the mechanism for large waveguide dispersion of theexemplary LP₁₁ spatial mode can be gained by considering the wavelengthdependence of the overlap of mode power with the fiber core. Aswavelength is increased toward cut-off of the spatial mode, mode powerof the higher order LP₁₁ spatial mode extends increasingly into thefiber cladding causing the mode to decrease its overlap of the core. Inturn, the group velocity increases because the cladding material has alower refractive index than the core material. When cut-off is reached,the higher order spatial mode resides substantially totally in thecladding so that the group velocity is determined substantially entirelyby the cladding material. As a result, large waveguide dispersion arisesfrom the rapid transition of mode power overlap from the core materialto the cladding material.

It is important to note that, while this mechanism causes the groupvelocity to increase with increasing wavelength, the waveguidedispersion is opposite in sign to the dispersion of conventional singlemode fiber supporting lightwave signals at a conventional transmissionwavelength such as λ_(op) equal to 1.55 μm. Moreover, because the rateof change of the spatial mode power overlap with the core increases asthe cut-off wavelength is approached, the higher order dispersion, thatis, the derivative of the dispersion with respect to wavelength, is alsoopposite in sign to the higher order dispersion in conventional singlemode fiber. This is a significant aspect of the instant invention andwill be discussed below.

Since this desirable dispersion affects the higher order spatial mode,it is necessary to insure that, in the exemplary embodiment, the lightentering the dispersive waveguide element is in the proper spatial mode.It is generally necessary to effect a mode conversion on the light sinceit usually is in the LP₀₁ spatial mode such as when it emerges from asingle mode optical fiber or dielectric waveguide or laser. Modeconversion is performed by a low loss, spatial mode converter such aselement 11 in FIG. 1 positioned at the input to the dispersive waveguideelement. The optical mode converter causes the spatial mode of thesignal light to be changed from the LP₀₁ spatial mode to the LP₁₁spatial mode, the latter higher order mode being supported by thedispersive waveguide element. Of course, output mode conversion from thedispersive waveguide element to a waveguide or other device is performedby optical mode converters 13 and 22 which cause the reciprocal changein spatial modes back to the fundamental or lower order LP₀₁ spatialmode.

The arrangement and number of the elements in the exemplary combination,namely, the mode converter and the dispersive waveguide, depend on thesystem application. As mentioned above, dispersion compensation may beperformed prior to launching the lightwave signal on the transmissionmedium, after receiving the lightwave signal from the transmissionmedium, and at one or more locations along the transmission mediumbetween the transmitter and receiver.

An exemplary embodiment for use in a compensation application at thereceiver in a lightwave communication system is shown in FIG. 1.Lightwave signals 14 are coupled from the transmission fiber into modeconverter 11 for conversion into a higher order spatial mode compatiblewith dispersive waveguide element 12. From mode converter 11, higherorder spatial mode lightwave signal 15 is coupled into dispersivewaveguide element 12 where a compensating amount of dispersion is addedto the lightwave signal. Output lightwave signal 16 from dispersivewaveguide element 12 is suitable for direct application to aphotodetection element or photoreceiver (neither shown in the drawing)to recover information from the lightwave signals.

Also shown in FIG. 1 is an optional embodiment in which optical modeconverter 13 is connected to the output of dispersive waveguide element12. Mode converter 13 (shown in dashed box to indicate that it is anoptional element) is utilized to convert the high order spatial mode oflightwave signal 16 into a lower or fundamental spatial mode forlightwave signal 17 compatible with a utilization means such as atransmission fiber or waveguide device. As configured in FIG. 1, theseelements are well suited for use along a transmission system tocompensate all or a portion of the dispersion from the transmissionmedium of the system. It is also contemplated that the optionalconfiguration of FIG. 1 is applicable at the transmitting or receivingsites where a light source such as a laser generates light or where aphotoreceiver accepts light, respectively, in the lower or fundamentalspatial mode.

When the lightwave signal is already available in the higher orderspatial mode, it is possible to utilize the configuration of elementsshown in FIG. 2. This occurs, for example, in a transmitting laser wherethe output light is available in the higher order spatial mode and it isdesired to predistort or precompensate the lightwave signal. As shown inFIG. 2, dispersive waveguide element 21 receives lightwave signal 23 fordispersion compensation. The lightwave signal is in the higher orderspatial mode supported by dispersive waveguide element 21. In order toobtain a lightwave signal which is compatible with the transmissionmedium, output lightwave signal 24 is coupled into optical modeconverter 22 for conversion into a lower order spatial mode compatiblewith the system transmission fiber.

As stated above, dispersion for the higher order modes is controlled asa function of wavelength. That is, the closer the operating wavelengthis to the cutoff wavelength, the larger the amount of dispersionavailable for providing compensation in the dispersive waveguideelement. Alternatively, as the operating and cutoff wavelengths tendfurther apart, there is less dispersion available for compensation. Inorder to quantify the extent to which the operating and cutoffwavelengths can differ and still allow effective compensation, it hasbeen found that the operating wavelength and the cutoff wavelengthdesirably differ by no more than about ten percent.

Chromatic dispersion in an optical fiber is often represented as the sumof two components, namely, waveguide dispersion and material dispersion.Waveguide dispersion relates to the contribution from the waveguidegeometry whereas material dispersion relates to the refractive indexvariation of glassy materials as a function of wavelength. Forstep-index silica-based optical fibers using the LP-mode approximation,waveguide dispersion is a function of the normalized refractive indexdifference between the core and cladding layers of the fiber, the coreradius, the speed of light, and a dimensionless function Ψ(V) dependentonly on the normalized frequency V. This relationship is well known topersons skilled in the art and is described in more detail in AppliedOptics, Vol. 10, pp. 2252 et seq. (1971). I should be noted that V, alsocalled the V-number, is a dimensionless parameter which determines howmany modes an optical fiber can support. For example, in step indexfibers, the number of guided modes is given approximately by the formulaN=V² /2 to include modes of both possible polarizations with bothchoices of sine and cosine functions.

In an example from experimental practice, the dispersive waveguideelement in FIGS. 1-3 was fabricated as a step index silica fiber havinga high index and a cutoff wavelength for the LP₁₁ spatial mode at 1.637umm. To achieve a large index step in the core of the fiber, a high Geconcentration was required in the core region. A core diameter of 4 μmwas employed in the step-index fiber. The relative effective indexdifference between the core and cladding layers was determined to beΔ=0.02. This fiber supported propagation of two spatial modes over thewavelength interval 1.050 μm to 1.637 μm. This fiber and its core Geconcentration were realized through standard fabrication and dopingtechniques known to those persons skilled in the optical fiber art. Forthe exemplary fiber supporting the LP₁₁ spatial mode at a wavelength of1.55 μm and using a V-number of 2.45, corresponding to Ψ(V)=20, it ispossible to compute a theoretical dispersion of -509 ps/nm·km. Actualmeasurements for this exemplary fiber indicate that this dispersivewaveguide element exhibits a maximum dispersion of -228 ps/nm·km for theLP₁₁ spatial mode at a wavelength of 1.56 μm. Such disparity between thepredicted theoretical dispersion and the actual observed dispersion ispossible from non-ideal shaping for the core index profile of the fiberand the like.

The spatial mode converter is also realizable in an optical fiberembodiment. By combining this embodiment with the fiber-based dispersivewaveguide element, it is possible to provide dispersion compensation inan all-fiber arrangement. One recently developed fiber-based modeconverter has been described in U.S. Pat. No. 4,974,931 being commonlyassigned herewith and whose description is expressly incorporated hereinby reference. This exemplary mode converter employed a two-mode fiber(that is, one supporting propagation of both the LP₀₁ spatial mode andthe LP₁₁ spatial mode over a particular wavelength interval) and alength of metallic wire such as gold wire. The length of wire waswrapped around an uncoated section of the two-mode fiber to create apredetermined number of turns under a particular tension. Brass chuckswere used to anchor the ends of the wire as solder points to maintainthe proper tension while the wrapped section of fiber was potted inepoxy. Spiral wrapping of the fiber causes circularly symmetricmicrobending of the fiber with a period which affects the wavelengthinterval over which spatial mode conversion is accomplished.

In the exemplary embodiment, the spatial mode converter was realized bya step index optical fiber different from the fiber for the dispersivewaveguide element and having a V-number of 3 at 1.55 μm to achieve abroadband response characteristic. It has been shown in J. LightwaveTechnology, Vol. 9, pp. 598-604 (1991) that the bandwidth for resonantcoupling between spatial modes at this larger V-number becomes large asa result of the group velocities between the LP₀₁ and LP₁₁ spatial modesbeing substantially equal. Wrapping the uncoated section of the fiber 81with twenty turns of 243 μm gold wire 82 as shown in FIG. 8 under 30 gtension completed device fabrication. The resulting exemplary modeconverter exhibited a full bandwidth at 90% conversion of 95 nm with apeak conversion to the LP₁₁ spatial mode of 96% at a wavelength of 1.55μm and a device insertion loss of 0.36 dB.

Operation of the dispersion compensation apparatus was verified usingnominally 7 ps optical pulses from a modelocked external cavity laser.The pulses were coupled into a single mode fiber having length 5.07 km.Chromatic dispersion in the single mode fiber caused broadening, of theoptical pulses by approximately an order of magnitude. After traversingthe single mode fiber, the dispersed optical pulse is coupled into thedispersion compensation arrangement. First, the spatial mode wasconverted from LP₀₁ to LP₁₁. Next, the higher order spatial mode of thedispersed optical pulse was input to the dispersive waveguide element,namely, the two-mode step-index fiber, having sufficient length anddispersion to effectively restore the original optical pulse shape. Inan effort to ensure excitation of the proper higher order spatial modein the dispersive waveguide element, an optical mode controller (element18 in FIG. 3) was inserted between the mode converter and the dispersivewaveguide element. In one embodiment, the spatial mode controllercomprised two loops (4 cm diameter) of optical fiber similar to thefiber in the dispersive waveguide element which was separately rotatablewith respect to the mode converter. Rotation of the mode controllerpermits adjustment of the spatial mode excited in the dispersivewaveguide element. Loss in this arrangement approached 5 dB withapproximately 2 dB attributable to fiber loss and the remaining lossarising from fusion splices between dissimilar fibers and insertion lossof the mode converter.

Results from the verification tests on the short optical pulses areshown in FIGS. 4 and 5. In each FIG., arbitrary power units for theinput optical pulse (solid line curve), the dispersed optical pulse(dashed line curve), and the restored or compensated optical pulse(dotted line curve) are plotted versus time units. For an operatingwavelength of 1.551 μm, complete restoration of the pulse shape wasachieved as shown in FIG. 4. In fact, some slight compression of therestored pulse indicates an over-compensation of the dispersion. For ashorter operating wavelength of 1.533 μm, the compensated pulse remainedslightly degraded from the input pulse indicative of under-compensationof the dispersion. From these results, it is clear that the dispersioncompensation arrangement substantially completely restores the pulseshape of dispersed optical pulses over a broad wavelength range.

To understand the importance of this arrangement, it is worthwhile toconsider a long haul transmission system such as a long distanceterrestrial (transcontinental) or transoceanic lightwave transmissionsystem. Using standard non-dispersion shifted, single mode fiber for thelong haul of approximately 10,000 km, it is possible to equalize thedispersion with an approximate length of 550 km of two-mode fiber as thehigher order dispersive waveguide element for the dispersioncompensation arrangement described above. That is, by compensatingdispersion in the higher order spatial mode, it is possible to use awaveguide element which is 1/20 the length of the overall lightwavetransmission system.

While examples of elements for use in the dispersion compensationarrangement have been described above, it is understood by personsskilled in the optical device art that other embodiments of the elementsare equally contemplated. For example, geometric and/or stressanisotropy can be introduced into an optical fiber to produce bimodal ortwo-mode operation. In one example, highly elliptical core fibers havebeen demonstrated to support propagation of the fundamental spatial modeand a higher order mode. See Kim et al., Opt. Lett., Vol. 12, No. 9,pp.729-731 (1987). Such fibers are useful in the realization of spatialmode converters and dispersive waveguide elements as described above.

There are many alternative embodiments for spatial mode converters, suchas those capable of converting optical signals from the LP₀₁ spatialmode to the LP₁₁ spatial mode. Taylor in J. Lightwave Technology, Vol.LT-2, No. 5, pp. 617-628 describes the use of spatially periodicmicrobending of anoptical fiber along the fiber axis to permit spatialmode conversion. Realization of such mode converters using spatiallyperiodic microbending has been described by Blake et al., Opt. Lett.,Vol. 11, No. 3, pp. 177-9 (1986) and Youngquist et al., Opt. Lett., Vol.9, No. 5, pp 177-9 (1984). Blake et al. wrap copper wires 61 and 62(0.025 cm which is approximately the length of fiber, i.e., modal beatlength, L_(B), over which the relative phase delay between the LP₀₁ andLP₁₁ spatial modes is 2π for the particular fiber used) around twocylinders 63 and 64, respectively, and bending the fiber 65 between thetwo rigid structures as shown in FIG. 6. Microbend spacing is shown tobe controlled by angling the fiber by a small amount relative to thewires. Youngquist et al. deform the fiber by mechanical stress appliedfrom a rigid crenelated plate structure where the period (combined ridgeand gap length) substantially matches the modal beat length. The fiberis placed between a rigid polished plate and the crenelated plate whilepressure is applied by clamping or screwing the plates together.

Wavelength selective mode conversion is also contemplated using aninline bimodal fiber tap as described by Hill et al., IEEE PhotonicsTechnology Lett., Vol. 2, No. 7, pp. 484-6 (1990). This device asdepicted in FIG. 7 employs a section of bimodal fiber 71 having aperiodic perturbation (stress induced holographically written or thelike) for performing resonant mode conversion from LP₀₁ to LP₁₁ at aparticular wavelength followed by a fused directional coupler 72 forseparating the fundamental mode (at other than the particularwavelength) from the higher order mode at the particular wavelength.Only the higher order mode is coupled into the secondary fiber 73 of thefused directional coupler 72.

Additional mode converters are realized by using a photoinduced gratingor holographically written grating on a two-mode optical fiber asdescribed by Park et al., Elect. Lett., Vol. 25, No. 12, pp. 797-9(1989). The grating period is substantially equal to the modal beatlength L_(B). By virtue of the grating, the refractive index of thefiber core undergoes a permanent change to permit spatial modeconversion. In a further embodiment, phase-matched evanescent modalcoupling is achieved in a device having a single mode fiber contacting abimodal fiber wherein the fibers are ground and polished to achieve thedesired transfer coupling. See Sorin et al., Opt. Lett., Vol. 11, No. 9,pp. 581-3 (1986).

While the use of multimode fibers in place of the bimodal fibers iscontemplated, such substitution can degrade performance of thedispersion compensation arrangement because of modal dispersion. Modaldispersion occurs when light is accidentally transferred from thedesired LP₁₁ spatial mode to another spatial mode such as thefundamental LP₀₁ spatial mode as the lightwave signal propagates alongthe fiber. Such coupling can occur as a result of imperfect splicesbetween fiber sections or through mechanical stresses put on the fiberin spooling and/or cabling. Since different spatial modes have differentvelocities, light that is coupled over to other than the desired spatialmode arrives at the fiber output at a different time from the light inthe desired mode thereby causing pulse broadening. This problem issolved in a bimodal step-index fiber that propagates both the LP₁₁ andLP₀₁ modes by adding an absorbing dopant to a small central region ofthe fiber core substantially along the longitudinal axis of doped fiber.Unpumped rare earth ions such as Er³⁺ are an exemplary absorbing dopantfor such an application in the wavelength region around 1.55 μm. Dopedcore region 93 is expected to be smaller than the core cross-section 92.With the absorbing region substantially at the center of the fiber core,the fiber suppresses the fundamental LP₀₁ spatial mode much morestrongly than the LP₁₁ mode since the fundamental mode has its intensitypeak substantially centralized in the core region whereas the LP₁₁spatial mode has an intensity null at the center of the core. Lightwhich is inadvertently coupled over to the unwanted fundamental modefrom the higher order mode is substantially completely absorbed andtherefore contributes only negligibly to optical pulse broadening. Thecriterion for effectively eliminating modal dispersion is that theabsorption length 1α, where α is the absorption coefficient, be muchsmaller than the coupling length for the modes which is thecharacteristic length over which power is transfer-red from one mode toanother.

Another modification contemplated for use in the dispersion compensationarrangement includes the provision of arbitrary optical gain by theaddition of rare earth dopant material such as Erbium to the fiber core.Since the Er³⁺ can be made to have a relatively small effect on thedispersive characteristics of the fiber, its distribution can beoptimized separately from dispersion considerations to provide thedesired amount of gain when pumped by an appropriate laser source. Anexemplary embodiment of this alternative in a step index fiber toprovide gain and to alleviate concomitantly the aforementionedmodal-dispersion is realizable by doping a small annular region 94 ofthe fiber core 92 to match the high intensity pattern for the higherorder spatial mode along the longitudinal axis of the optical fiber. Theannular distribution of dopant material provides preferential gain forthe higher order spatial mode over the fundamental spatial mode. Thisoccurs because the latter has most of its intensity located at thecenter of the fiber core whereas the former has its intensitydistributed in an annular region 94 outside the core center. Because ofthe greatly reduced gain for the fundamental mode, light which isaccidentally coupled over to the fundamental mode exhibits a far lowerpower at the fiber output relative to higher order spatial mode light.In this way, the effects of modal dispersion are again overcome.

An advantage of the latter embodiment is that, where the pump laserlight wavelength is close enough to the signal light wavelength andwhere the mode converter has a broadband response, the pump light canalso be coupled into the doped fiber in the higher order spatial modealong with the signal light to provide optimum spatial overlap with theannular Er³⁺ doped core region for efficient pumping. Close wavelengthspacing is observed for 1.480 μm pump light for Er-doped fiberamplifiers and 1.550 μm signal light.

As disclosed above, dispersion compensating (DC) fiber can be designedsuch that " . . . the higher order dispersion, that is, the derivativeof the dispersion with respect to the wavelength, is also opposite insign to the higher order dispersion in conventional single mode fiber."This offers the possibility of further improvement in system operatingcharacteristics, namely, the possibility of relatively low chromaticdispersion over a relatively wide wavelength range, such that signalscan be transmittal at high rate over a multiplicity of WDM channels.

More specifically, the invention is embodied in an optical fibercommunication system that comprises, in addition to a length L (notnecessarily a continuous length) of conventional single mode (SM) fiber,a length L_(DC) (also not necessarily a continuous length) of DC fiber.Those skilled in the art will appreciate that L could be the length ofconventional fiber between transmitter and receiver, or that it could bethe length of conventional fiber in any appropriate portion of thetransmission path, e.g., the length of a span from regenerator (oroptical amplifier) to regenerator (or amplifier).

Typically L is much larger than L_(DC), desirably L≧5L_(DC) or even10L_(DC). The DC fiber not only has chromatic dispersion at apredetermined wavelength λ_(op) (typically about 1.5 μm) of the oppositesign as the chromatic dispersion of the SM fiber at λ_(op) but also hasa derivative with respect to wavelength of the chromatic dispersion thathas the opposite sign as the derivative of the chromatic dispersion ofthe SM fiber over a range of wavelengths that typically includes λ_(op).Chromatic dispersion of the conventional SM and the DC fiber will bedesignated D and D_(DC), respectively, and the derivative of thechromatic dispersion with respect to λ will be designated D' andD'_(DC), respectively. Typically D and D' are positive at about 1.5 μm.

The DC fiber is designed, and L_(DC) is selected such that, typically,LD+L_(DC) ·D_(DC) ˜0 at λ_(op), and such that, typically, the absolutevalue of (LD+L_(DC) ·D_(DC))/(L+L_(DC)) is less than a predeterminedvalue (e.g. less than 1.5 ps/nm·km) for all wavelengths in a wavelengthrange (exemplarily at least about 50 nm) that includes λ_(op). Thelatter condition can be met if -D'_(DC) ˜(L/L_(DC))·D' at λ_(op). Forinstance, a step index DC fiber (core diameter 4 μm, effective indexdifference between core and cladding 2%, cut off wavelength of LP₁₁1.640 μm) had about -228 ps/nm·km chromatic dispersion at 1.56 μm.Joining a length L_(DC) of the DC fiber to a length L (L_(DC) =L/13.2)of conventional single mode fiber (AT&T's 5D SM fiber) resulted in acombination that had dispersion of absolute value less than 0.4 ps/nm·kmover the wavelength interval from 1.53-1.56 μm, and substantially lessthan 1.5 ps/nm·km over a 50 nm wavelength region that includes 1.56 μm.See also C. D. Poole, et al., post deadline paper digest, Optical FiberConference, San Jose, Calif., Feb. 6, 1992, and C. D. Poole et al.,Optics Lett., Vol. 17 (14), p. 985, Jul. 5, 1992, and Poole et al.,Proceedings for European Conf. on Optical Communication ECOC'92, Berlin,Germany, Volume 3, paper PDI.4, "Elliptical-Core Dual-Mode FiberDispersion Compensator," all incorporated herein by reference.

Those skilled in the art will also appreciate that practice of theinstant invention does not require use of the above disclosed DC fiberand mode conversion. Indeed, any DC fiber that yields the requiredD'_(DC) of appropriate sign is contemplated for use according to theinvention. Exemplarily, such a fiber is disclosed in co-assigned U.S.patent application Ser. No. 978,002, filed Nov. 18, 1992 for A. M.Vengsarkar, et al., entitled "Article Comprising aDispersion-Compensating Optical Waveguide", incorporated herein byreference.

FIG. 11 schematically depicts an exemplary optical fiber communicationsystem according to the invention wherein numeral 120 refers to signalradiation emitted by transmitter 110 which is guided in transmissionfiber 130 to optical amplifier/dispersion compensator 140, is thentypically guided through further lengths of transmission fiber andfurther amplifiers/compensators (not shown) to receiver 160. Exemplarilythe amplifier/compensator comprises a length of DC fiber 170 and EDFA180. Pump radiation 190 is provided to combination 140 to be coupledinto the EDFA. It will be appreciated that the functions of compensationand amplification could be performed by rare earth-doped DC fiber.

I claim:
 1. An article comprising a length L_(DC) of optical fiber (saidfiber to be designated "DC" fiber) that supports propagation ofradiation of wavelength λ_(op), where λ_(op) ˜1.5 μm, and furthercomprising a length L of optical fiber that is single mode fiber forradiation of wavelength λ_(op), that has chromatic dispersion D(λ) atwavelength λ and that furthermore has a derivative with respect towavelength D(λ)' of the chromatic dispersion at wavelengthλ;CHARACTERIZED IN THAT the DC fiber has chromatic dispersion D_(DC) (λ)at wavelength and furthermore has a derivative with respect towavelength D'_(DC) (λ) of the chromatic dispersion at wavelength λ, withD_(DC) (λ) and D'_(DC) (λ) being of the opposite sign as D(λ) and D'(λ),respectively, at λ=λ_(op).
 2. The article of claim 1, wherein the DCfiber is selected such that L_(DC) ·D_(DC) (λ)+L·D(λ) is approximatelyequal to zero at λ_(op), and where in L is much greater than L_(DC). 3.The article of claim 2, wherein the DC fiber is further-more selectedsuch that the absolute value of [L_(DC) ·D_(DC) (λ)+L·D(λ)]/(L_(DC) +L)is less than 1.5 ps/nm·km for all wavelengths in a wavelength range thatis at least 50 nm and includes λ_(op).
 4. The article of claim 1,wherein the DC fiber is selected such that D'_(DC) (λ) is essentiallyequal to -(L/L_(DC))D'(λ) at λ=λ_(op).
 5. The article of claim 1,wherein the article is an optical fiber communication system thatcomprises a transmitter, a receiver, and an optical fiber transmissionpath signal-transmissively connecting the transmitter and the receiver,the transmission path comprising said lengths of fiber L_(DC) and L, andwherein L>5L_(DC).
 6. The article of claim 5, wherein L_(DC) +L issubstantially the total length of the transmission path.
 7. The articleof claim 5, wherein the optical fiber communication system furthercompromises an optical amplifier intermediate the transmitter and thereceiver.
 8. The article of claim 7, comprising a plurality of opticalamplifiers intermediate the transmitter and the receiver, wherein L_(DC)+L is substantially the total length of the transmission path between agiven optical amplifier and an optical amplifier adjacent to the givenone.
 9. The article of claim 1, compromising first means for convertinga lightwave signal of wavelength λ_(op) from a first spatial mode to asecond spatial mode, the second spatial mode being a higher orderspatial mode than the first spatial mode; wherein the DC fiber supportspropagation of the lightwave signal substantially in the second spatialmode and is connected to an output of the first converting means. 10.The article of claim 1, wherein said optical fiber that is single modefiber for radiation of wavelength λ_(op) has zero chromatic dispersionat a wavelength less that λ_(op).
 11. The article of claim 10, whereinthe article is an optical fiber communication system, and wherein saidwavelength less than λ_(op) is about 1.3 μm.